If you're using test or development licenses (such as those bundled with TechNet or MSDN subscriptions), I would suggest not activating the installation unless you specifically need to use the test server beyond the evaluation period.
The main reason is that Microsoft usually does not allow reactivation of used TechNet or MSDN licenses. [Edited based on comment]
If you are using regular retail licenses, this is less of an issue, except that activating the new machine will likely involve a call to Microsoft to document the license transfer.
The following option probably does not apply to you, but it's worth mentioning for the benefit of others: If you are using volume licenses, then activation is not ever necessary, but you should let your license administrator know of the change, so that your organization remains in compliance with the terms of the volume license agreement.